In fused salt electrolysis of aluminium, an electrolytic cell comprises a bath made of iron sheet or steel, whose base is lined with a thermal insulation. Located in this bath are a plurality of adjacently disposed cathode blocks which are sealed by refractory lining material and form the cell base. The conversion of the electrolytic bath to aluminium takes place in this melting bath under the action of the electric current. This reaction generally takes place at a temperature of above 950° C.
In order to meet the requirements for the thermal and chemical resistance and to ensure the required electrical conductivity, the cathode blocks are made of carbon-containing materials, which can be semi-graphitic to graphitic. These materials are formed by extrusion or vibration compaction after thorough mixing of the basic materials. In this case, a mixture of pitch, calcined anthracite and/or graphite can be used which is then baked at about 1200° C. The semi-graphitic cathode produced from these mixtures is generally designated as carbon cathode. A mixture of pitch and coke with or without added graphite can also be used. In this case the materials are baked at about 800° C. and then graphite-annealed at above 2400° C.
This graphitic cathode thus produced is called graphite cathode.
The carbon cathodes conventionally used have only moderate electrical and thermal properties, which no longer satisfy the operating conditions of modern cells, in particular those having a high current intensity. The need to reduce the energy consumption, in particular in existing installations, has promoted the use of graphite cathodes.
The graphite annealing treatment of the graphite cathode at above 2400° C. makes it possible to increase the electrical and thermal conductivity with the result that conditions are created which contribute to an optimised operation of an electrolytic cell. As a result of the reduction in the electrical resistance of the cathode, the energy consumption decreases. In addition, the intensity of the current introduced into the cell can be increased, with the result that an increase in aluminium production is possible. The high thermal conductivity of the cathode then allows the excess heat generated as a result of the increase in the current intensity to be led off. In addition, the graphite cathode cells are electrically less unstable than the carbon cathode cells, i.e. they exhibit a smaller fluctuation of the electrical potentials.
However, it has been found that cells equipped with graphite cathodes have a shorter lifetime than cells equipped with carbon cathodes. This is because the erosion rate of a graphite cathode block is significantly higher than that of a carbon cathode block.
The low erosion resistance of a graphite cathode block is therefore its weak point and the problem arises of increasing the erosion resistance and therefore the lifetime of the graphite cathode blocks.
In addition to the extensive wear of the cathode block, the formation of local conically shaped holes in the cathode base is a serious problem. These holes grow from the surface of the cathode in the direction of the busbar. If the busbar is reached, this results in a substantial increase in the iron content of the aluminium and ultimately in destruction of the cell.